Transformers are electrical devices commonly used to transform power from an AC source to an AC load. They may also be used to electrically isolate the supply from the load by providing adequate insulation and creepage distance between the primary and secondary windings to electrically isolate one winding from the other. Transformers may also be used to generate high voltage and are called step-up type transformers when used to increase voltage on the secondary winding as related to the primary winding. Transformers consist of an input or primary winding, an output or secondary winding, and a means of magnetically coupling the primary winding to the secondary winding through a magnetic material called a core and/or through the air. The primary and secondary windings are typically wound using wire called magnet wire which consists of a conductive wire with a thin insulated coating to keep adjacent winding turns from shorting together. The relationship between the input power to the primary winding and output power from the secondary winding is referred to as the coupling coefficient. The coupling coefficient can be varied through several parameters including change in the transformer's core material or core size, change in winding material, change in the number of winding turns or size of the wire used for the winding. While the effect on the coupling coefficient is minimal in almost all types of transformers, the coupling coefficient can also be changed through proximity of the primary windings to the core, proximity of the core to the secondary windings, and through proximity of the primary windings to the secondary windings. Transformers are typically referred to as AC devices but can be used in a DC pulse application where an electric pulse into a transformer's primary winding causes a change in the magnetic field allowing the transformer to function.
In DC pulse applications, the transformer is typically constructed with a magnetic core and the transformer behaves similar to the AC application provided the pulse current does not become so high the magnetic core is pushed beyond saturation. Pushing the transformer's core beyond saturation in DC pulse type applications such as DC to DC converters is not common because it causes the transformer to become inefficient and results in over-heating. However, in high current DC pulse type applications such as capacitive discharge circuits, saturation of the core is common provided the duration of input pulse current is small relative to the time between the pulses. The relation between the duration of input pulse current to time between input current pulses is called the duty cycle. AC transformers that operate continuously have a duty cycle of 100%. A DC pulse type transformer used in a high current application where there is a long time between the pulse currents (typically while a capacitor is charging) relative to the duration of the pulse current (or during the capacitor's discharge), the duty cycle would be significantly lower than 100%. Due to the lower duty cycle, a transformer used in a high current DC pulse type application has time to cool between pulses and can be significantly smaller than a transformer designed to deliver the same peak output power continuously at a 100% duty cycle. The smaller size and the high currents associated with capacitive discharge type circuits cause the high current DC pulse type transformer's magnetic core to become saturated such that very little increase in output power from the secondary winding is delivered for an increase in input power to the primary winding due to the transformer's magnetic coupling through the transformers magnetic core.
While the magnetic coupling becomes a limiting factor the farther and farther the core is pushed beyond saturation, other changes can be made in the transformer's construction such as the physical proximity of the primary windings in relation to the secondary windings to increase the coupling coefficient and improve efficiency. The farther the transformer's core is pushed beyond saturation, the more dependent the coupling coefficient becomes on the primary and secondary winding's proximity to each other.
Transformers are typically constructed using one of two construction methods, stick wound construction or bobbin construction. The stick wound construction typically consists of an insulated rectangular core tube (typically cardboard) with multiple layers of windings wound around the core tube with a layer of insulation between each layer of windings. Both the primary and secondary windings are typically wound in several layers and separated from each other by a layer of insulation where all the primary winding layers are either inside or outside all of the secondary winding layers. Depending on the dielectric requirements between the primary and secondary windings and between each layer of windings within the primary and secondary, insulation between any two winding layers using materials that are commonly available may be a couple or few thousandths of an inch thick and can be a thin as 0.001 inches. In addition to providing insulation, the layers of insulation function as a mechanical form to keep the windings in layers and in position such that the transformer windings keep their shape and do not fall apart.
After the windings are wound, the coil must be lead-set which consists of finding the ends of the wires between the layers of insulation, pulling the ends of the wires out of the coil, placing insulated tubes over the wires, striping the insulated coating off the ends of the magnet wire, and terminating the ends of the wires. This lead-setting is a labor intensive and expensive process for one primary winding and one secondary winding where both ends of both windings are lead-set. To increase the coupling coefficient, the primary and secondary windings can be wound in alternating layers where multiple layers of primary windings are used to separate multiple layers of secondary windings resulting in a closer proximity between the primary winding turns and secondary winding turns. However, the number of wire ends that need to be lead-set goes up with the number of windings along with the cost.
In step-up transformers used to generate high voltage, the number of layers of secondary windings increases due to the increase in the number of turns in the secondary winding along with the number of layers of insulation necessary between the secondary winding layers. Without the use of multiple layers of primary windings separating multiple layers of secondary windings, the typical construction with all the primary winding layers inside or outside all of the secondary winding layers results in the insulation between all the layers of the secondary winding layers separating each secondary layer farther and farther as each layer moves away from the primary winding. While the secondary winding layer closest to the primary winding is separated from the primary winding by one layer of insulation, the second closest secondary winding layer is separated by two layers of insulation, the third closest secondary winding layer by three layers, and so on. This cumulative number of insulation layers between the secondary winding layers that are farthest away from the primary winding results in an average distance between the primary winding turns and secondary winding turns that significantly limits the coupling coefficient of the transformer when the transformer is used in a high current pulse type application where the transformer's core is pushed beyond saturation and where the coupling coefficient becomes more and more dependent on the primary and secondary winding's proximity to each other.
The bobbin construction transformer consists of a bobbin used as a form for the windings to be wound around rather than the core tube. The bobbin is typically injection molded and includes a round or rectangular tube (called a barrel) with a wall at each end (called a flange). The winding turns are wound around the outside perimeter of the bobbin's barrel between the flanges in an area called a winding bay. The bobbin's barrel and flanges create the structure for the windings to keep a particular shape and allow for multiple layers of windings without insulation between the layers as in the stick wound construction. The bobbin may be provided with pins or connectors for winding machines to wrap both ends of the wires around and terminate the wires, eliminating the lead-setting operation and associated labor costs. In high voltage step-up transformers requiring a high number of secondary turns where the transformer's secondary output voltage potential is higher than the dielectric strength of the coating on the magnet wire used for the secondary winding, the bobbin is typically provided with multiple winding bays to divide secondary output voltage between several winding bays, each winding bay able to withstand part of the secondary winding's total output voltage potential. The multi-winding bay bobbin provides a cost-effective manufacturing construction without the labor intensive costs associated with the stick wound construction.
The bobbin construction transformer can be constructed for a high voltage application using a single bobbin, where the primary and secondary windings are provided side-by-side on the same bobbin, where the bobbin is provided with a flange between the primary and secondary windings, and where the secondary is provided in multiple winding bays. However, this single bobbin side-by-side coil construction causes a significant separation between several of the primary winding turns and most of the secondary winding turns resulting in an extremely inefficient transformer the transformer is a high voltage, high current DC pulse type transformer where the core is pushed beyond saturation.
To place more secondary winding turns in close proximity to the primary winding turns, two bobbins are typically used where the primary winding is wound on one bobbin, the secondary winding is wound on a second bobbin that provides multiple winding bays, and one bobbin and winding are placed inside the other bobbin and winding. While the multiple winding bays for the secondary winding allow the secondary winding turns to be in close proximity to each other, the barrel of the bobbin used for the winding on the outside results in the primary and secondary windings being physically separated by the thickness of the barrel. The most common material used to injection mold bobbins is nylon which requires a wall section of approximately 1/32 inches thick to mold. Materials other than nylon are available to injection mold bobbins, however, these materials cost much more and still require thicknesses close to nylon or must be molded using a wall section thicker than nylon. In addition to the thick insulation associated with the dual bobbin construction that physically separate the primary and secondary winding turns from each other, additional distance is required between the primary and secondary windings due to assembly purposes. To allow assembly of one bobbin over the other bobbin after the winding on the inside bobbin is wound, typically 0.005 to 0.010 inches is required between the inside of the outer bobbin and the outside of the coil on the inner bobbin adding to the distance physically separating the primary and secondary windings.
In the high voltage, high current DC pulse type transformer where the core is pushed beyond saturation and where the primary winding's turns physical proximity to the secondary winding turns becomes more critical for a high coupling coefficient, both the stick wound and bobbin constructions have distinct advantages and disadvantages. While the stick wound construction may use insulation films as thin as 1/1000 inches between each of the winding layers and allow some of the primary winding layers to be in close proximity to some of the secondary winding layers, the high number of secondary winding layers and the insulation required between each layer necessary for the construction results in a significant separation between several of the primary winding turns and the secondary winding turns unless multiple primary winding layers are used to separate the layers of the secondary winding. While the bobbin construction eliminates the need for the multiple layers of insulation between each winding layer, the number of winding bays necessary for a high voltage secondary winding on the same bobbin as the primary winding results in a significant separation between the primary winding turns and secondary winding turns. Even in the dual bobbin construction with one bobbin over the other bobbin, while more secondary winding turns are in close proximity to the primary winding turns, the primary and secondary windings are separated by the thickness of the barrel of the outside bobbin plus additional distance necessary for assembly purposes. Thus it would be beneficial to provide a high voltage, high current, DC pulse type transformer where the core is pushed beyond saturation with a secondary winding provided in a plurality of winding bays as in the bobbin construction and with the primary and secondary windings in close proximity to each other separated by only a thin layer of insulation as in the stick wound construction.